The present invention relates to real time monitoring of the condition of a fluid, such as engine lubricant during engine operation, by impedance spectroscopy wherein electrodes are immersed in the lubricant and excited with a relatively low voltage alternating current at frequencies indicative of the bulk impedance of the fluid and separately at frequencies indicative of the electrode surface properties. The use of impedance spectroscopy for fluid condition monitoring by the aforesaid method is known and described in U.S. Pat. No. 6,278,281 issued to Bauer, et al., assigned to the assignee of the present application.
The method of determining the condition of a fluid, and particularly lubricants, described in the aforesaid patent is subject to shifting as the temperature of the fluid varies in service and can introduce significant errors to the analysis of the probe signal. Heretofore, temperature compensation of the current measurements in order to provide an accurate indication of the bulk and electrode surface impedance, has proven difficult and therefore the determination of the impedance of the fluid rendered suspect with wide variations in the temperature during operation.
It has been desired to provide a way or means for determining the condition of a fluid by impedance spectroscopy derived from current measurements of a probe excited with an alternating current voltage in a manner which provides accurate correction the effects of temperature variation in the fluid and additionally provides sufficient resolution to yield a correct correlation of the fluid condition with values of impedance for known fluid conditions.